Using iMove to export videos into MPeg4 and .mov results in a black/blank movie

A colleague and I are attempting to create a screencast using video created with Snapz Pro X. We successfully, eventually, managed to delete the appropriate frames and merge the clips together (to the best of our knowledge) and so we decided to export the movie.

We were somewhat less cautious than might have been sensible given it was the first time we were using this program but as it had taken him a fair amount of hours to get the movie into a useful state it's not surprising. We clicked file > export movie... and then I believe we got a pop-up asking us to choose QuickTime or the other option - which is the default. He clicked export and so our movie was exported in MPeg 4 format.

When we tried to then open the movie it opened in iTunes but, though it played, the whole movie was just black as if every frame was a black picture. It was precisely the correct length, just black.

We wondered whether it was a mistake we made so we went back to iMovie and clicked 'export to QuickTime' this time around and it did indeed open with QuickTime but again it was black.

I thought it might be that the movie had already been saved in the black MPeg 4 state and so we had just exported the black movie to QuickTime. I'm fairly sure that's the wrong answer and it doesn't help us understand why it exported a black movie in the first place.

Any help will be much appreciated. Sorry for the long question - I thought it would be better to give you as many details up front. If we do work it out, I'll post the answer back here too in case someone else does the same things wrong that we did.

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